Composition of isolated synaptic boutons reveals the amounts of vesicle trafficking proteins

Synaptic vesicle-omics in mice captures signatures of aging and synucleinopathy

Neurotransmitter release occurs through exocytosis of synaptic vesicles. α-Synuclein's function and dysfunction in Parkinson's disease and other synucleinopathies is thought to be tightly linked to synaptic vesicle binding. Age is the biggest risk factor for synucleinopathy, and ~15% of synaptic vesicle proteins have been linked to central nervous system diseases. Yet, age- and disease-induced changes in synaptic vesicles remain unexplored. Via systematic analysis of synaptic vesicles at the ultrastructural, protein, and lipid levels, we reveal specific changes in synaptic vesicle populations, proteins, and lipids over age in wild-type mice and in α-synuclein knockout mice with and without expression of human α-synuclein. Strikingly, we find several previously undescribed synaptic changes in mice lacking α-synuclein, suggesting that loss of α-synuclein function contributes to synaptic dysfunction. These findings not only provide insights into synaptic vesicle biology and disease mechanisms in synucleinopathy, but also serve as a baseline for further mechanistic exploration of age- and disease-related alterations in synaptic vesicles.

Synaptophysin accelerates synaptic vesicle fusion by expanding the membrane upon neurotransmitter loading

Synaptic transmission is mediated by the exocytotic release of neurotransmitters stored in synaptic vesicles (SVs). SVs filled with neurotransmitters preferentially undergo exocytosis, but it is unclear how this is achieved. Here, we show that during transmitter loading, SVs substantially increase in size, which is reversible and requires synaptophysin, an abundant membrane protein with an unclear function. SVs are larger when synaptophysin is knocked out, and conversely, liposomes are smaller when reconstituted with synaptophysin. Moreover, transmitter loading of SVs accelerates fusion in vitro, which is abolished when synaptophysin is lacking despite near normal transmitter uptake. We conclude that synaptophysin functions as a curvature-promoting entity in the SV membrane, allowing for major lateral expansion of the SV membrane during neurotransmitter filling, thus increasing their propensity for exocytosis.

Extracellular vesicles from hiPSC-derived NSCs protect human neurons against Aβ-42 oligomers induced neurodegeneration, mitochondrial dysfunction and tau phosphorylation

BACKGROUND

Alzheimer's disease (AD) is characterized by the accumulation of amyloid beta-42 (Aβ-42) in the brain, causing various adverse effects. Thus, therapies that reduce Aβ-42 toxicity in AD are of great interest. One promising approach is to use extracellular vesicles from human induced pluripotent stem cell-derived neural stem cells (hiPSC-NSC-EVs) because they carry multiple therapeutic miRNAs and proteins capable of protecting neurons against Aβ-42-induced toxicity. Therefore, this in vitro study investigated the proficiency of hiPSC-NSC-EVs to protect human neurons from Aβ-42 oligomers (Aβ-42o) induced neurodegeneration.

METHODS

We isolated hiPSC-NSC-EVs using chromatographic methods and characterized their size, ultrastructure, expression of EV-specific markers and proficiency in getting incorporated into mature human neurons. Next, mature human neurons differentiated from two different hiPSC lines were exposed to 1 µM Aβ-42o alone or with varying concentrations of hiPSC-NSC-EVs. The protective effects of hiPSC-NSC-EVs against Aβ-42o-induced neurodegeneration, oxidative stress, mitochondrial dysfunction, impaired autophagy, and tau phosphorylation were ascertained using multiple measures and one-way ANOVA with Newman-Keuls multiple comparisons post hoc tests.

RESULTS

A significant neurodegeneration was observed when human neurons were exposed to Aβ-42o alone. Neurodegeneration was associated with (1) elevated levels of reactive oxygen species (ROS), mitochondrial superoxide, malondialdehyde (MDA) and protein carbonyls (PCs), (2) increased expression of proapoptotic Bax and Bad genes and proteins, and genes encoding mitochondrial complex proteins, (3) diminished mitochondrial membrane potential and mitochondria, (4) reduced expression of the antiapoptotic gene and protein Bcl-2, and autophagy-related proteins, and (5) increased phosphorylation of tau. However, the addition of an optimal dose of hiPSC-NSC-EVs (6 × 109 EVs) to human neuronal cultures exposed to Aβ-42o significantly reduced the extent of neurodegeneration, along with diminished levels of ROS, superoxide, MDA and PCs, normalized expressions of Bax, Bad, and Bcl-2, and autophagy-related proteins, higher mitochondrial membrane potential and mitochondria, enhanced expression of genes linked to mitochondrial complex proteins, and reduced tau phosphorylation.

CONCLUSIONS

An optimal dose of hiPSC-NSC-EVs could significantly decrease the degeneration of human neurons induced by Aβ-42o. The results support further research into the effectiveness of hiPSC-NSC-EVs in AD, particularly their proficiency in preserving neurons and slowing disease progression.

Integrated multi-omic characterizations of the synapse reveal RNA processing factors and ubiquitin ligases associated with neurodevelopmental disorders

The molecular composition of the excitatory synapse is incompletely defined due to its dynamic nature across developmental stages and neuronal populations. To address this gap, we apply proteomic mass spectrometry to characterize the synapse in multiple biological models, including the fetal human brain and human induced pluripotent stem cell (hiPSC)-derived neurons. To prioritize the identified proteins, we develop an orthogonal multi-omic screen of genomic, transcriptomic, interactomic, and structural data. This data-driven framework identifies proteins with key molecular features intrinsic to the synapse, including characteristic patterns of biophysical interactions and cross-tissue expression. The multi-omic analysis captures synaptic proteins across developmental stages and experimental systems, including 493 synaptic candidates supported by proteomics. We further investigate three such proteins that are associated with neurodevelopmental disorders-Cullin 3 (CUL3), DEAD-box helicase 3 X-linked (DDX3X), and Y-box binding protein-1 (YBX1)-by mapping their networks of physically interacting synapse proteins or transcripts. Our study demonstrates the potential of an integrated multi-omic approach to more comprehensively resolve the synaptic architecture.

In silico nanoscope to study the interplay of genome organization and transcription regulation

In eukaryotic genomes, regulated access and communication between cis-regulatory elements (CREs) are necessary for enhancer-mediated transcription of genes. The molecular framework of the chromatin organization underlying such communication remains poorly understood. To better understand it, we develop a multiscale modeling pipeline to build near-atomistic models of the 200 kb Nanog gene locus in mouse embryonic stem cells comprising nucleosomes, transcription factors, co-activators, and RNA polymerase II-mediator complexes. By integrating diverse experimental data, including protein localization, genomic interaction frequencies, cryo-electron microscopy, and single-molecule fluorescence studies, our model offers novel insights into chromatin organization and its role in enhancer-promoter communication. The models equilibrated by high-performance molecular dynamics simulations span a scale of ∼350 nm, revealing an experimentally consistent local and global organization of chromatin and transcriptional machinery. Our models elucidate that the sequence-regulated chromatin accessibility facilitates the recruitment of transcription regulatory proteins exclusively at CREs, guided by the contrasting nucleosome organization compared to other regions. By constructing an experimentally consistent near-atomic model of chromatin in the cellular environment, our approach provides a robust framework for future studies on nuclear compartmentalization, chromatin organization, and transcription regulation.

The sodium leak channel NALCN is regulated by neuronal SNARE complex proteins

NALCN (sodium leak channel, nonselective) is vital for regulating electrical activity in neurons and other excitable cells, and mutations in the channel or its auxiliary proteins lead to severe neurodevelopmental disorders. Here, we show that the neuronal SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) complex proteins syntaxin and SNAP25 (synaptosome-associated protein 25), which enable synaptic transmission in the nervous system, inhibit the activity of the NALCN channel complex in both heterologous systems and primary neurons. The existence of this interaction suggests that the neurotransmitter release machinery can regulate electrical signaling directly and therefore modulate the threshold for its own activity. We further find that reduction of NALCN currents is sufficient to promote cell survival in syntaxin-depleted cells. This suggests that disinhibited NALCN may cause the puzzling phenomenon of rapid neuronal cell death in the absence of syntaxin. This interaction could offer opportunities for future drug development against genetic diseases linked to both NALCN- and SNARE protein-containing complexes.

Conserved transcription factors coordinate synaptic gene expression through repression

Chemical synapses are the primary sites of communication in the nervous system. Synapse formation is a complex process involving hundreds of proteins that must be expressed in two cells at the same time. We find that synaptic genes are broadly and specifically coordinated at the level of transcription across developing nervous systems. How this spatiotemporal coordination is achieved remains an open question. Through genomic and functional studies in Drosophila, we demonstrate corresponding coordination of chromatin accessibility and identify chromatin regulators DEAF1 and CLAMP as broad repressors of synaptic gene expression outside windows of peak synaptogenesis. Disruption of either factor temporally dysregulates synaptic gene expression across neuronal subtypes, leading to excess synapse formation. We further find that DEAF1, which is linked to syndromic intellectual disability, is both necessary and sufficient to constrain synapse formation. Our findings reveal the critical importance of broad temporally coordinated repression of synaptic gene expression in regulating neuronal connectivity and identify two key repressors.

The α-synuclein seed amplification assay: Interpreting a test of Parkinson's pathology

The α-synuclein seed amplification assay (αSyn-SAA) sensitively detects Lewy pathology, the amyloid state of α-synuclein, in the cerebrospinal fluid (CSF) of patients with Parkinson's disease (PD). The αSyn-SAA harnesses the physics of seeding, whereby a superconcentrated solution of recombinant α-synuclein lowers the thermodynamic threshold (nucleation barrier) for aggregated α-synuclein to act as a nucleation catalyst ("seed") to trigger the precipitation (nucleation) of monomeric α-synuclein into pathology. This laboratory setup increases the signal for identifying a catalyst if one is present in the tissue examined. The result is binary: positive, meaning precipitation occurred, and a catalyst is present, or negative, meaning no precipitation, therefore no catalyst. Since protein precipitation via seeding can only occur at a concentration many-fold higher than the human brain, laboratory-elicited seeding does not mean human brain seeding. We suggest that a positive αSyn-SAA reveals the presence of pathological α-synuclein but not the underlying etiology for the precipitation of monomeric α-synuclein into its pathological form. Thus, a positive αSyn-SAA supports a clinical diagnosis of PD but cannot inform disease pathogenesis, ascertain severity, predict the rate of progression, define biology or biological subtypes, or monitor treatment response.

Understanding the molecular diversity of synapses

Synapses are composed of thousands of proteins, providing the potential for extensive molecular diversity to shape synapse type-specific functional specializations. In this Review, we explore the landscape of synaptic diversity and describe the mechanisms that expand the molecular complexity of synapses, from the genotype to the regulation of gene expression to the production of specific proteoforms and the formation of localized protein complexes. We emphasize the importance of examining every molecular layer and adopting a systems perspective to understand how these interconnected mechanisms shape the diverse functional and structural properties of synapses. We explore current frameworks for classifying synapses and methodologies for investigating different synapse types at varying scales, from synapse-type-specific proteomics to advanced imaging techniques with single-synapse resolution. We highlight the potential of synapse-type-specific approaches for integrating molecular data with cellular functions, circuit organization and organismal phenotypes to enable a more holistic exploration of neuronal phenomena across different scales.

Independently engaging protein tethers of different length enhance synaptic vesicle trafficking to the plasma membrane

Synaptic vesicle (SV) trafficking toward the plasma membrane (PM) and subsequent SV maturation are essential for neurotransmitter release. These processes, including SV docking and priming, are co-ordinated by various proteins, such as SNAREs, Munc13 and synaptotagmin (Syt), which connect (tether) the SV to the PM. Here, we investigated how tethers of varying lengths mediate SV docking using a simplified mathematical model. The heights of the three tether types, as estimated from the structures of the SNARE complex, Munc13 and Syt, defined the SV-PM distance ranges for tether formation. Geometric considerations linked SV-PM distances to the probability and rate of tether formation. We assumed that SV tethering constrains SV motility and that multiple tethers are associated by independent interactions. The model predicted that forming multiple tethers favours shorter SV-PM distances. Although tethers acted independently in the model, their geometrical properties often caused their sequential assembly, from longer ones (Munc13/Syt), which accelerated SV movement towards the PM, to shorter ones (SNAREs), which stabilized PM-proximal SVs. Modifying tether lengths or numbers affected SV trafficking. The independent implementation of tethering proteins enabled their selective removal to mimic gene knockout (KO) situations. This showed that simulated SV-PM distance distributions qualitatively aligned with published electron microscopy studies upon removal of SNARE and Syt tethers, whereas Munc13 KO data were best approximated when assuming additional disruption of SNARE tethers. Thus, although salient features of SV docking can be accounted for by independent tethering alone, our results suggest that functional tether interactions not yet featured in our model are crucial for biological function. KEY POINTS: A mathematical model describing the role of synaptic protein tethers to localize transmitter-containing vesicles is developed based on geometrical considerations and structural information of synaptotagmin, Munc13 and SNARE proteins. Vesicle movement, along with tether association and dissociation, are modelled as stochastic processes, with tethers functioning independently of each other. Multiple tethers cooperate to recruit vesicles to the plasma membrane and keep them there: Munc13 and Syt as the longer tethers accelerate the movement towards the membrane, whereas short SNARE tethers stabilize them there. Model predictions for situations in which individual tethers are removed agree with the results from experimental studies upon gene knockout. Changing tether length or copy numbers affects vesicle trafficking and steady-state distributions.

Spatially resolved DNP-assisted NMR illuminates the conformational ensemble of α-synuclein in intact viable cells

The protein α-syn adopts a wide variety of conformations including an intrinsically disordered monomeric form and an α-helical rich membrane-associated form that is thought to play an important role in cellular membrane processes. However, despite the high affinity of α-syn for membranes, evidence that the α-helical form is adopted inside cells has been indirect. DNP-assisted solid state NMR on frozen cellular samples can report on protein conformations inside cells. Moreover, by controlling the distribution of the DNP polarization agent throughout the cellular biomass, such experiments can provide quantitative information upon the entire structural ensemble or provide information about spatially resolved sub-populations. Using DNP-assisted magic angle spinning (MAS) NMR we establish that purified α-syn in the membrane-associated and intrinsically disordered forms have distinguishable spectra. We then introduced isotopically labeled monomeric α-syn into cells. When the DNP polarization agent is dispersed homogenously throughout the cell, we found that a minority of the α-syn inside cells adopted a highly α-helical rich conformation. When the DNP polarization agent is peripherally localized, we found that the α-helical rich conformation predominates. Thus, we provide direct evidence that α-helix rich conformations of α-syn are adopted near the cellular periphery inside cells under physiological conditions. Moreover, we demonstrate how selectively altering the spatial distribution of the DNP polarization agent can be a powerful tool to observe spatially distinct structural ensembles. This approach paves the way for more nuanced investigations into the conformations that proteins adopt in different areas of the cell.

Purification of Afference-Specific Synaptosome Populations Using Fluorescence-Activated Synaptosome Sorting

The central nervous system contains a complex intermingled network of neuronal, glial, and vascular cells, and for several decades, neurobiologists have used subcellular fractionation methods to analyze the molecular structure and functional features of the different cell populations. Biochemists have optimized fractionation protocols that enrich specific compartments such as synapses (called "synaptosomes") and synaptic vesicles to reduce this complexity. However, these approaches suffered from a lack of specificity and purity, which is why we previously extended the conventional synaptosome preparation to purify fluorescent synaptosomes from VGLUT1venus knock-in mice on a cell sorter. We adapted our previous protocol to sort from single neuronal projections and small target regions of the brain as we did in the present example by labeling dopaminergic projections to the striatum. We proved that our newest method allows a steep enrichment in fluorescent dopaminergic synaptosomes containing presynaptic varicosities and associated postsynaptic elements and a substantial depletion in glial contaminants. Here we propose a detailed procedure for implementing projection-specific fluorescence-activated synaptosome sorting.

Control of Synaptotagmin-1 Trafficking by SV2A-Mechanism and Consequences for Presynaptic Function and Dysfunction

Synaptic vesicle protein 2A (SV2A) is an abundant synaptic vesicle cargo with an as yet unconfirmed role in presynaptic function. It is also heavily implicated in epilepsy, firstly being the target of the leading anti-seizure medication levetiracetam and secondly with loss of function mutations culminating in human disease. A range of potential presynaptic functions have been proposed for SV2A; however its interaction with the calcium sensor for synchronous neurotransmitter release, synaptotagmin-1 (Syt1), has received particular attention over the past decade. In this review we will assess the evidence that the primary role of SV2A is to control the expression and localisation of Syt1 at the presynapse. This will integrate biochemical, cell biological and physiological studies where the interaction, trafficking and functional output of Syt1 is altered by SV2A. The potential for SV2A-dependent epilepsy to be a result of dysfunctional Syt1 expression and localisation is also discussed. Finally, a series of key open questions will be posed that require resolution before a definitive role for SV2A in Syt1 function in health and disease can be confirmed.

N-acetylation of α-synuclein enhances synaptic vesicle clustering mediated by α-synuclein and lysophosphatidylcholine

Previously, we reported that α-synuclein (α-syn) clusters synaptic vesicles (SV) Diao et al., 2013, and neutral phospholipid lysophosphatidylcholine (LPC) can mediate this clustering Lai et al., 2023. Meanwhile, post-translational modifications (PTMs) of α-syn such as acetylation and phosphorylation play important yet distinct roles in regulating α-syn conformation, membrane binding, and amyloid aggregation. However, how PTMs regulate α-syn function in presynaptic terminals remains unclear. Here, based on our previous findings, we further demonstrate that N-terminal acetylation, which occurs under physiological conditions and is irreversible in mammalian cells, significantly enhances the functional activity of α-syn in clustering SVs. Mechanistic studies reveal that this enhancement is caused by the N-acetylation-promoted insertion of α-syn's N-terminus and increased intermolecular interactions on the LPC-containing membrane. N-acetylation in our work is shown to fine-tune the interaction between α-syn and LPC, mediating α-syn's role in synaptic vesicle clustering.

Molecular architecture of synaptic vesicles

Synaptic vesicles (SVs) store and transport neurotransmitters to the presynaptic active zone for release by exocytosis. After release, SV proteins and excess membrane are recycled via endocytosis, and new SVs can be formed in a clathrin-dependent manner. This process maintains complex molecular composition of SVs through multiple recycling rounds. Previous studies explored the molecular composition of SVs through proteomic analysis and fluorescent microscopy, proposing a model for an average SV (1). However, the structural heterogeneity and molecular architecture of individual SVs are not well described. Here, we used cryoelectron tomography to visualize molecular details of SVs isolated from mouse brains and inside cultured neurons. We describe several classes of small proteins on the SV surface and long proteinaceous densities inside SVs. We identified V-ATPases, determined a structure using subtomogram averaging, and showed them forming a complex with the membrane-embedded protein synaptophysin (Syp). Our bioluminescence assay revealed pairwise interactions between vesicle-associated membrane protein 2 and Syp and V-ATPase Voe1 domains. Interestingly, V-ATPases were randomly distributed on the surface of SVs irrespective of vesicle size. A subpopulation of isolated vesicles and vesicles inside neurons contained a partially assembled clathrin coat with an icosahedral symmetry. We observed V-ATPases under clathrin cages in several isolated clathrin-coated vesicles (CCVs). Additionally, from isolated SV preparations and within hippocampal neurons we identified clathrin baskets without vesicles. We determined their and CCVs preferential location in proximity to the cell membrane. Our analysis advances the understanding of individual SVs' diversity and their molecular architecture.

Morphology and intervesicle distances in condensates of synaptic vesicles and synapsin

Synaptic vesicle clusters or pools are functionally important constituents of chemical synapses. In the so-called reserve and the active pools, neurotransmitter-loaded synaptic vesicles (SVs) are stored and conditioned for fusion with the synaptic membrane and subsequent neurotransmitter release during synaptic activity. Vesicle clusters can be considered as so-called membraneless compartments, which form by liquid-liquid phase separation. Synapsin as one of the most abundant synaptic proteins has been identified as a major driver of pool formation. It has been shown to induce liquid-liquid phase separation and form condensates on its own in solution, but also has been shown to integrate vesicles into condensates in vitro. In this process, the intrinsically disordered region of synapsin is believed to play a critical role. Here, we first investigate the solution structure of synapsin and SVs separately by small-angle x-ray scattering. In the limit of low momentum transfer q, the scattering curve for synapsin gives clear indication for supramolecular aggregation (condensation). We then study mixtures of SVs and synapsin-forming condensates, aiming at the morphology and intervesicle distances, i.e., the structure of the condensates in solution. To obtain the structure factor S(q) quantifying intervesicle correlation, we divide the scattering curve of condensates by that of pure SV suspensions. Analysis of S(q) in combination with numerical simulations of cluster aggregation indicates a noncompact fractal-like vesicular fluid with rather short intervesicle distances at the contact sites.

Quantitative single-molecule analysis of assembly and Ca2+-dependent disassembly of synaptotagmin oligomers on lipid bilayers

Synaptotagmin-1 (Syt-1) self-assembles into ring-like oligomers, and genetic and biochemical evidence suggest that oligomerization is needed to clamp synaptic vesicles and stabilize them for Ca2+-evoked release. However, oligomerization has not yet been demonstrated on lipid bilayers or studied in quantitative biophysical terms. Here we utilize single-molecule imaging methods to monitor the assembly and disassembly of oligomeric clusters of Syt-1 on lipid bilayers in real-time. Syt-1 assembled into two distinct classes of oligomers, small (5 ± 2 subunits) and large (15 ± 2 subunits). Each class assembled at a constant kon that was always proportional to its ultimate size, but both classes disassembled at the same unit rate (koff) independent of its size. Both large and small oligomers explosively disassembled when Ca2+ was added. The F349A mutation in the Syt-1 nearly eliminates the large class of oligomers but does not reduce the small class. Altogether, the physical-chemical properties of Syt-1 oligomers meet or exceed the physiologic requirements to function as such a clamp.

MJF-14 proximity ligation assay detects early non-inclusion alpha-synuclein pathology with enhanced specificity and sensitivity

α-Synuclein proximity ligation assay (PLA) has proved a sensitive technique for detection of non-Lewy body α-synuclein aggregate pathology. Here, we describe the MJF-14 PLA, a new PLA towards aggregated α-synuclein with unprecedented specificity, using the aggregate-selective α-synuclein antibody MJFR-14-6-4-2 (hereafter MJF-14). Signal in the assay correlates with α-synuclein aggregation in cell culture and human neurons, induced by α-synuclein overexpression or pre-formed fibrils. Co-labelling of MJF-14 PLA and pS129-α-synuclein immunofluorescence in post-mortem cases of dementia with Lewy bodies shows that while the MJF-14 PLA reveals extensive non-inclusion pathology, it is not sensitive towards pS129-α-synuclein-positive Lewy bodies. In Parkinson's disease brain, direct comparison of PLA and immunohistochemistry with the MJF-14 antibody shows widespread α-synuclein pathology preceding the formation of conventional Lewy pathology. In conclusion, we introduce an improved α-synuclein aggregate PLA to uncover abundant non-inclusion pathology, which deserves future validation with brain bank resources and in different synucleinopathies.

Novel tools to quantify total, phospho-Ser129 and aggregated alpha-synuclein in the mouse brain

Assays for quantifying aggregated and phosphorylated (S129) human α-synuclein protein are widely used to evaluate pathological burden in patients suffering from synucleinopathy disorders. Many of these assays, however, do not cross-react with mouse α-synuclein or exhibit poor sensitivity for this target, which is problematic considering the preponderance of mouse models at the forefront of pre-clinical α-synuclein research. In this project, we addressed this unmet need by reformulating two existing AlphaLISA® SureFire® Ultra™ total and pS129 α-synuclein assay kits to yield robust and ultrasensitive (LLoQ ≤ 0.5 pg/mL) quantification of mouse and human wild-type and pS129 α-synuclein protein. We then employed these assays, together with the BioLegend α-synuclein aggregate ELISA, to assess α-synuclein S129 phosphorylation and aggregation in different mouse brain tissue preparations. Overall, we highlight the compatibility of these new immunoassays with rodent models and demonstrate their potential to advance knowledge surrounding α-synuclein phosphorylation and aggregation in synucleinopathies.

Morphological correlates of synaptic protein turnover in the mouse brain

Synaptic proteins need to be replaced regularly, to maintain function and to prevent damage. It is unclear whether this process, known as protein turnover, relates to synaptic morphology. To test this, we relied on nanoscale secondary ion mass spectrometry, to detect newly synthesized synaptic components in the brains of young adult (6 mo old) and aged mice (24 mo old), and on transmission electron microscopy, to reveal synapse morphology. Several parameters correlated to turnover, including pre- and postsynaptic size, the number of synaptic vesicles and the presence of a postsynaptic nascent zone. In aged mice, the turnover of all brain compartments was reduced by ∼20%. The turnover rates of the pre- and postsynapses correlated well in aged mice, suggesting that they are subject to common regulatory mechanisms. This correlation was poorer in young adult mice, in line with their higher synaptic dynamics. We conclude that synapse turnover is reflected by synaptic morphology.

β-synuclein regulates the phase transitions and amyloid conversion of α-synuclein

Parkinson's disease (PD) and Dementia with Lewy Bodies (DLB) are neurodegenerative disorders characterized by the accumulation of α-synuclein aggregates. α-synuclein forms droplets via liquid-liquid phase separation (LLPS), followed by liquid-solid phase separation (LSPS) to form amyloids, how this process is physiologically-regulated remains unclear. β-synuclein colocalizes with α-synuclein in presynaptic terminals. Here, we report that β-synuclein partitions into α-synuclein condensates promotes the LLPS, and slows down LSPS of α-synuclein, while disease-associated β-synuclein mutations lose these capacities. Exogenous β-synuclein improves the movement defects and prolongs the lifespan of an α-synuclein-expressing NL5901 Caenorhabditis elegans strain, while disease-associated β-synuclein mutants aggravate the symptoms. Decapeptides targeted at the α-/β-synuclein interaction sites are rationally designed, which suppress the LSPS of α-synuclein, rescue the movement defects, and prolong the lifespan of C. elegans NL5901. Together, we unveil a Yin-Yang balance between α- and β-synuclein underlying the normal and disease states of PD and DLB with therapeutical potentials.

The synaptic vesicle cluster as a controller of pre- and postsynaptic structure and function

The synaptic vesicle cluster (SVC) is an essential component of chemical synapses, which provides neurotransmitter-loaded vesicles during synaptic activity, at the same time as also controlling the local concentrations of numerous exo- and endocytosis cofactors. In addition, the SVC hosts molecules that participate in other aspects of synaptic function, from cytoskeletal components to adhesion proteins, and affects the location and function of organelles such as mitochondria and the endoplasmic reticulum. We argue here that these features extend the functional involvement of the SVC in synapse formation, signalling and plasticity, as well as synapse stabilization and metabolism. We also propose that changes in the size of the SVC coalesce with changes in the postsynaptic compartment, supporting the interplay between pre- and postsynaptic dynamics. Thereby, the SVC could be seen as an 'all-in-one' regulator of synaptic structure and function, which should be investigated in more detail, to reveal molecular mechanisms that control synaptic function and heterogeneity.

Amyloid accelerator polyphosphate fits as the mystery density in α-synuclein fibrils

Aberrant aggregation of α-Synuclein is the pathological hallmark of a set of neurodegenerative diseases termed synucleinopathies. Recent advances in cryo-electron microscopy have led to the structural determination of the first synucleinopathy-derived α-Synuclein fibrils, which contain a non-proteinaceous, "mystery density" at the core of the protofilaments, hypothesized to be highly negatively charged. Guided by previous studies that demonstrated that polyphosphate (polyP), a universally conserved polyanion, significantly accelerates α-Synuclein fibril formation, we conducted blind docking and molecular dynamics simulation experiments to model the polyP binding site in α-Synuclein fibrils. Here, we demonstrate that our models uniformly place polyP into the lysine-rich pocket, which coordinates the mystery density in patient-derived fibrils. Subsequent in vitro studies and experiments in cells revealed that substitution of the 2 critical lysine residues K43 and K45 with alanine residues leads to a loss of all previously reported effects of polyP binding on α-Synuclein, including stimulation of fibril formation, change in filament conformation and stability as well as alleviation of cytotoxicity. In summary, our study demonstrates that polyP fits the unknown electron density present in in vivo α-Synuclein fibrils and suggests that polyP exerts its functions by neutralizing charge repulsion between neighboring lysine residues.

SV2A controls the surface nanoclustering and endocytic recruitment of Syt1 during synaptic vesicle recycling

Following exocytosis, the recapture of plasma membrane-stranded vesicular proteins into recycling synaptic vesicles (SVs) is essential for sustaining neurotransmission. Surface clustering of vesicular proteins has been proposed to act as a 'pre-assembly' mechanism for endocytosis that ensures high-fidelity retrieval of SV cargo. Here, we used single-molecule imaging to examine the nanoclustering of synaptotagmin-1 (Syt1) and synaptic vesicle protein 2A (SV2A) in hippocampal neurons. Syt1 forms surface nanoclusters through the interaction of its C2B domain with SV2A, which are sensitive to mutations in this domain (Syt1K326A/K328A) and SV2A knockdown. SV2A co-clustering with Syt1 is reduced by blocking SV2A's cognate interaction with Syt1 (SV2AT84A). Surprisingly, impairing SV2A-Syt1 nanoclustering enhanced the plasma membrane recruitment of key endocytic protein dynamin-1, causing accelerated Syt1 endocytosis, altered intracellular sorting and decreased trafficking of Syt1 to Rab5-positive endocytic compartments. Therefore, SV2A and Syt1 are segregated from the endocytic machinery in surface nanoclusters, limiting dynamin recruitment and negatively regulating Syt1 entry into recycling SVs.

⍺-Synuclein levels in Parkinson's disease - Cell types and forms that contribute to pathogenesis

Parkinson's disease (PD) has two main pathological hallmarks, the loss of nigral dopamine neurons and the proteinaceous aggregations of ⍺-synuclein (⍺Syn) in neuronal Lewy pathology. These two co-existing features suggest a causative association between ⍺Syn aggregation and the underpinning mechanism of neuronal degeneration in PD. Both increased levels and post-translational modifications of ⍺Syn can contribute to the formation of pathological aggregations of ⍺Syn in neurons. Recent studies have shown that the protein is also expressed by multiple types of non-neuronal cells in the brain and peripheral tissues, suggesting additional roles of the protein and potential diversity in non-neuronal pathogenic triggers. It is important to determine (1) the threshold levels triggering ⍺Syn to convert from a biological to a pathologic form in different brain cells in PD; (2) the dominant form of pathologic ⍺Syn and the associated post-translational modification of the protein in each cell type involved in PD; and (3) the cell type associated biological processes impacted by pathologic ⍺Syn in PD. This review integrates these aspects and speculates on potential pathological mechanisms and their impact on neuronal and non-neuronal ⍺Syn in the brains of patients with PD.

Exploring the structural dynamics of the vesicle priming machinery

Various cell types release neurotransmitters, hormones and many other compounds that are stored in secretory vesicles by exocytosis via the formation of a fusion pore traversing the vesicular membrane and the plasma membrane. This process of membrane fusion is mediated by the Soluble N-ethylmaleimide-Sensitive Factor Attachment Proteins REceptor (SNARE) protein complex, which in neurons and neuroendocrine cells is composed of the vesicular SNARE protein Synaptobrevin and the plasma membrane proteins Syntaxin and SNAP25 (Synaptosomal-Associated Protein of 25 kDa). Before a vesicle can undergo fusion and release of its contents, it must dock at the plasma membrane and undergo a process named 'priming', which makes it ready for release. The primed vesicles form the readily releasable pool, from which they can be rapidly released in response to stimulation. The stimulus is an increase in Ca2+ concentration near the fusion site, which is sensed primarily by the vesicular Ca2+ sensor Synaptotagmin. Vesicle priming involves at least the SNARE proteins as well as Synaptotagmin and the accessory proteins Munc18, Munc13, and Complexin but additional proteins may also participate in this process. This review discusses the current views of the interactions and the structural changes that occur among the proteins of the vesicle priming machinery.

Actomyosin-mediated inhibition of synaptic vesicle release under CB1R activation

Long-term synaptic plasticity is critical for adaptive function of the brain, but presynaptic mechanisms of functional plasticity remain poorly understood. Here, we show that changes in synaptic efficacy induced by activation of the cannabinoid type-1 receptor (CB1R), one of the most widespread G-protein coupled receptors in the brain, requires contractility of the neuronal actomyosin cytoskeleton. Specifically, using a synaptophysin-pHluorin probe (sypH2), we show that inhibitors of non-muscle myosin II (NMII) ATPase as well as one of its upstream effectors Rho-associated kinase (ROCK) prevent the reduction of synaptic vesicle release induced by CB1R activation. Using 3D STORM super-resolution microscopy, we find that activation of CB1R induces a redistribution of synaptic vesicles within presynaptic boutons in an actomyosin dependent manner, leading to vesicle clustering within the bouton and depletion of synaptic vesicles from the active zone. We further show, using sypH2, that inhibitors of NMII and ROCK specifically restore the release of the readily releasable pool of synaptic vesicles from the inhibition induced by CB1R activation. Finally, using slice electrophysiology, we find that activation of both NMII and ROCK is necessary for the long-term, but not the short-term, form of CB1R induced synaptic plasticity at excitatory cortico-striatal synapses. We thus propose a novel mechanism underlying CB1R-induced plasticity, whereby CB1R activation leads to a contraction of the actomyosin cytoskeleton inducing a reorganization of the functional presynaptic vesicle pool, preventing vesicle release and inducing long-term depression.

N-acetylation of α-synuclein enhances synaptic vesicle clustering mediated by α-synuclein and lysophosphatidylcholine

Post-translational modifications (PTMs) of α-synuclein (α-syn) such as acetylation and phosphorylation play important yet distinct roles in regulating α-syn conformation, membrane binding, and amyloid aggregation. However, how PTMs regulate α-syn function in presynaptic terminals remains unclear. Previously, we reported that α-syn clusters synaptic vesicles (SV)1, and neutral phospholipid lysophosphatidylcholine (LPC) can mediate this clustering2. Here, based on our previous findings, we further demonstrate that N-terminal acetylation, which occurs under physiological conditions and is irreversible in mammalian cells, significantly enhances the functional activity of α-syn in clustering SVs. Mechanistic studies reveal that this enhancement is caused by the N-acetylation-promoted insertion of α-syn's N-terminus and increased intermolecular interactions on the LPC-containing membrane. Our work demonstrates that N-acetylation fine-tunes α-syn-LPC interaction for mediating α-syn's function in SV clustering.

APP fragment controls both ionotropic and non-ionotropic signaling of NMDA receptors

NMDA receptors (NMDARs) are ionotropic receptors crucial for brain information processing. Yet, evidence also supports an ion-flux-independent signaling mode mediating synaptic long-term depression (LTD) and spine shrinkage. Here, we identify AETA (Aη), an amyloid-β precursor protein (APP) cleavage product, as an NMDAR modulator with the unique dual regulatory capacity to impact both signaling modes. AETA inhibits ionotropic NMDAR activity by competing with the co-agonist and induces an intracellular conformational modification of GluN1 subunits. This favors non-ionotropic NMDAR signaling leading to enhanced LTD and favors spine shrinkage. Endogenously, AETA production is increased by in vivo chemogenetically induced neuronal activity. Genetic deletion of AETA production alters NMDAR transmission and prevents LTD, phenotypes rescued by acute exogenous AETA application. This genetic deletion also impairs contextual fear memory. Our findings demonstrate AETA-dependent NMDAR activation (ADNA), characterizing AETA as a unique type of endogenous NMDAR modulator that exerts bidirectional control over NMDAR signaling and associated information processing.

Intermediate steps in the formation of neuronal SNARE complexes

Neuronal exocytosis requires the assembly of three SNARE proteins, syntaxin and SNAP25 on the plasma membrane and synaptobrevin on the vesicle membrane. However, the precise steps in this process and the points at which assembly and fusion are controlled by regulatory proteins are unclear. In the present work, we examine the kinetics and intermediate states during SNARE assembly in vitro using a combination of time resolved fluorescence and EPR spectroscopy. We show that syntaxin rapidly forms a dimer prior to forming the kinetically stable 2:1 syntaxin:SNAP25 complex and that the 2:1 complex is not diminished by the presence of excess SNAP25. Moreover, the 2:1 complex is temperature-dependent with a reduced concentration at 37 °C. The two segments of SNAP25 behave differently. The N-terminal SN1 segment of SNAP25 exhibits a pronounced increase in backbone ordering from the N- to the C-terminus that is not seen in the C-terminal SNAP25 segment SN2. Both the SN1 and SN2 segments of SNAP25 will assemble with syntaxin; however, while the association of the SN1 segment with syntaxin produces a stable 2:2 (SN1:syntaxin) complex, the complex formed between SN2 and syntaxin is largely disordered. Synaptobrevin fails to bind syntaxin alone but will associate with syntaxin in the presence of either the SN1 or SN2 segments; however, the synaptobrevin:syntaxin:SN2 complex remains disordered. Taken together, these data suggest that synaptobrevin and syntaxin do not assemble in the absence of SNAP25 and that the SN2 segment of SNAP25 is the last to enter the SNARE complex.

Dynamin 1xA interacts with Endophilin A1 via its spliced long C-terminus for ultrafast endocytosis

Dynamin 1 mediates fission of endocytic synaptic vesicles in the brain and has two major splice variants, Dyn1xA and Dyn1xB, which are nearly identical apart from the extended C-terminal region of Dyn1xA. Despite a similar set of binding partners, only Dyn1xA is enriched at endocytic zones and accelerates vesicle fission during ultrafast endocytosis. Here, we report that Dyn1xA achieves this localization by preferentially binding to Endophilin A1 through a newly defined binding site within its long C-terminal tail extension. Endophilin A1 binds this site at higher affinity than the previously reported site, and the affinity is determined by amino acids within the Dyn1xA tail but outside the binding site. This interaction is regulated by the phosphorylation state of two serine residues specific to the Dyn1xA variant. Dyn1xA and Endophilin A1 colocalize in patches near the active zone, and mutations disrupting Endophilin A binding to the long tail cause Dyn1xA mislocalization and stalled endocytic pits on the plasma membrane during ultrafast endocytosis. Together, these data suggest that the specificity for ultrafast endocytosis is defined by the phosphorylation-regulated interaction of Endophilin A1 with the C-terminal extension of Dyn1xA.

VAMP2 regulates phase separation of α-synuclein

α-Synuclein (αSYN), a pivotal synaptic protein implicated in synucleinopathies such as Parkinson's disease and Lewy body dementia, undergoes protein phase separation. We reveal that vesicle-associated membrane protein 2 (VAMP2) orchestrates αSYN phase separation both in vitro and in cells. Electrostatic interactions, specifically mediated by VAMP2 via its juxtamembrane domain and the αSYN C-terminal region, drive phase separation. Condensate formation is specific for R-SNARE VAMP2 and dependent on αSYN lipid membrane binding. Our results delineate a regulatory mechanism for αSYN phase separation in cells. Furthermore, we show that αSYN condensates sequester vesicles and attract complexin-1 and -2, thus supporting a role in synaptic physiology and pathophysiology.

Extracellular Vesicles from hiPSC-derived NSCs Protect Human Neurons against Aβ-42 Oligomers Induced Neurodegeneration, Mitochondrial Dysfunction and Tau Phosphorylation

Background

One of the hallmarks of Alzheimer's disease (AD) is the buildup of amyloid beta-42 (Aβ-42) in the brain, which leads to various adverse effects. Therefore, therapeutic interventions proficient in reducing Aβ-42-induced toxicity in AD are of great interest. One promising approach is to use extracellular vesicles from human induced pluripotent stem cell-derived neural stem cells (hiPSC-NSC-EVs) because they carry multiple therapeutic miRNAs and proteins capable of protecting neurons against Aβ-42-induced pathological changes. Therefore, this in vitro study investigated the proficiency of hiPSC-NSC-EVs to protect human neurons derived from two distinct hiPSC lines from Aβ-42o-induced neurodegeneration.

Methods

We isolated hiPSC-NSC-EVs using chromatographic methods and characterized their size, ultrastructure, expression of EV-specific markers and proficiency in getting incorporated into mature human neurons. Next, mature human neurons differentiated from two different hiPSC lines were exposed to 1 µM Aβ-42 oligomers (Aβ-42o) alone or with varying concentrations of hiPSC-NSC-EVs. The protective effects of hiPSC-NSC-EVs against Aβ-42o-induced neurodegeneration, increased oxidative stress, mitochondrial dysfunction, impaired autophagy, and tau phosphorylation were ascertained using multiple measures and one-way ANOVA with Newman-Keuls multiple comparisons post hoc tests.

Results

Significant neurodegeneration was observed when human neurons were exposed to Aβ-42o alone. Notably, neurodegeneration was associated with elevated levels of oxidative stress markers malondialdehyde (MDA) and protein carbonyls (PCs), increased expression of proapoptotic Bax and Bad genes and proteins, reduced expression of the antiapoptotic gene and protein Bcl-2, increased expression of genes encoding mitochondrial complex proteins, decreased expression of autophagy-related proteins Beclin-1 and microtubule-associated protein 1 light chain 3B, and increased phosphorylation of tau. However, the addition of an optimal dose of hiPSC-NSC-EVs (6 x 10 9 EVs) to human neuronal cultures exposed to Aβ-42o significantly reduced the extent of neurodegeneration, along with diminished levels of MDA and PCs, normalized expressions of Bax, Bad, and Bcl-2, and genes linked to mitochondrial complex proteins, and reduced tau phosphorylation.

Conclusions

The findings demonstrate that an optimal dose of hiPSC-NSC-EVs could significantly decrease the degeneration of human neurons induced by Aβ-42o. The results also support further research into the effectiveness of hiPSC-NSC-EVs in AD, particularly their proficiency in preserving neurons and slowing disease progression.

The components of an electrical synapse as revealed by expansion microscopy of a single synaptic contact

Most nervous systems combine both transmitter-mediated and direct cell-cell communication, known as 'chemical' and 'electrical' synapses, respectively. Chemical synapses can be identified by their multiple structural components. Electrical synapses are, on the other hand, generally defined by the presence of a 'gap junction' (a cluster of intercellular channels) between two neuronal processes. However, while gap junctions provide the communicating mechanism, it is unknown whether electrical transmission requires the contribution of additional cellular structures. We investigated this question at identifiable single synaptic contacts on the zebrafish Mauthner cells, at which gap junctions coexist with specializations for neurotransmitter release and where the contact unequivocally defines the anatomical limits of a synapse. Expansion microscopy of these single contacts revealed a detailed map of the incidence and spatial distribution of proteins pertaining to various synaptic structures. Multiple gap junctions of variable size were identified by the presence of their molecular components. Remarkably, most of the synaptic contact's surface was occupied by interleaving gap junctions and components of adherens junctions, suggesting a close functional association between these two structures. In contrast, glutamate receptors were confined to small peripheral portions of the contact, indicating that most of the synaptic area functions as an electrical synapse. Thus, our results revealed the overarching organization of an electrical synapse that operates with not one, but multiple gap junctions, in close association with structural and signaling molecules known to be components of adherens junctions. The relationship between these intercellular structures will aid in establishing the boundaries of electrical synapses found throughout animal connectomes and provide insight into the structural organization and functional diversity of electrical synapses.

CHARMM-GUI Multicomponent Assembler for modeling and simulation of complex multicomponent systems

Atomic-scale molecular modeling and simulation are powerful tools for computational biology. However, constructing models with large, densely packed molecules, non-water solvents, or with combinations of multiple biomembranes, polymers, and nanomaterials remains challenging and requires significant time and expertise. Furthermore, existing tools do not support such assemblies under the periodic boundary conditions (PBC) necessary for molecular simulation. Here, we describe Multicomponent Assembler in CHARMM-GUI that automates complex molecular assembly and simulation input preparation under the PBC. In this work, we demonstrate its versatility by preparing 6 challenging systems with varying density of large components: (1) solvated proteins, (2) solvated proteins with a pre-equilibrated membrane, (3) solvated proteins with a sheet-like nanomaterial, (4) solvated proteins with a sheet-like polymer, (5) a mixed membrane-nanomaterial system, and (6) a sheet-like polymer with gaseous solvent. Multicomponent Assembler is expected to be a unique cyberinfrastructure to study complex interactions between small molecules, biomacromolecules, polymers, and nanomaterials.

The αSynuclein half-life conundrum

αSynuclein (αSyn) misfolding and aggregation frequently precedes neuronal loss associated with Parkinson's Disease (PD) and other Synucleinopathies. The progressive buildup of pathological αSyn species results from alterations on αSyn gene and protein sequence, increased local concentrations, variations in αSyn interactome and protein network. Therefore, under physiological conditions, it is mandatory to regulate αSyn proteostasis as an equilibrium among synthesis, trafficking, degradation and extracellular release. In this frame, a crucial parameter is protein half-life. It provides indications of the turnover of a specific protein and depends on mRNA synthesis and translation regulation, subcellular localization, function and clearance by the designated degradative pathways. For αSyn, the molecular mechanisms regulating its proteostasis in neurons have been extensively investigated in various cellular models, either using biochemical or imaging approaches. Nevertheless, a converging estimate of αSyn half-life has not emerged yet. Here, we discuss the challenges in studying αSyn proteostasis under physiological and pathological conditions, the advantages and disadvantages of the experimental strategies proposed so far, and the relevance of determining αSyn half-life from a translational perspective.

SGIP1 binding to the α-helical H9 domain of cannabinoid receptor 1 promotes axonal surface expression

Endocannabinoid signalling mediated by cannabinoid receptor 1 (CB1R, also known as CNR1) is critical for homeostatic neuromodulation of both excitatory and inhibitory synapses. This requires highly polarised axonal surface expression of CB1R, but how this is achieved remains unclear. We previously reported that the α-helical H9 domain in the intracellular C terminus of CB1R contributes to axonal surface expression by an unknown mechanism. Here, we show in rat primary neuronal cultures that the H9 domain binds to the endocytic adaptor protein SGIP1 to promote CB1R expression in the axonal membrane. Overexpression of SGIP1 increases CB1R axonal surface localisation but has no effect on CB1R lacking the H9 domain (CB1RΔH9). Conversely, SGIP1 knockdown reduces axonal surface expression of CB1R but does not affect CB1RΔH9. Furthermore, SGIP1 knockdown diminishes CB1R-mediated inhibition of presynaptic Ca2+ influx in response to neuronal activity. Taken together, these data advance mechanistic understanding of endocannabinoid signalling by demonstrating that SGIP1 interaction with the H9 domain underpins axonal CB1R surface expression to regulate presynaptic responsiveness.

Unique Properties of Synaptosomes and Prospects for Their Use for the Treatment of Alzheimer's Disease

Alzheimer's disease (AD) is a severe neurodegenerative condition affecting millions worldwide. Prevalence of AD correlates with increased life expectancy and aging population in the developed countries. Considering that AD is a multifactorial disease involving various pathological processes such as synaptic dysfunction, neuroinflammation, oxidative stress, and improper protein folding, a comprehensive approach targeting multiple pathways may prove effective in slowing the disease progression. Cellular therapy and its further development in the form of cell vesicle and particularly mitochondrial transplantation represent promising approaches for treating neurodegeneration. The use of synaptosomes, due to uniqueness of their contents, could mark a new stage in the development of comprehensive therapies for neurodegenerative diseases, particularly AD. Synaptosomes contain unique memory mitochondria, which differ not only in size but also in functionality compared to the mitochondria in the neuronal soma. These synaptosomal mitochondria actively participate in cellular communication and signal transmission within synapses. Synaptosomes also contain other elements such as their own protein synthesis machinery, synaptic vesicles with neurotransmitters, synaptic adhesion molecules, and microRNAs - all crucial for synaptic transmission and, consequently, cognitive processes. Complex molecular ensemble ensures maintenance of the synaptic autonomy of mitochondria. Additionally, synaptosomes, with their affinity for neurons, can serve as an optimal platform for targeted drug delivery to nerve cells. This review discusses unique composition of synaptosomes, their capabilities and advantages, as well as limitations of their suggested use as therapeutic agents for treating neurodegenerative pathologies, particularly AD.

Presynaptic nanoscale components of retrograde synaptic signaling

While our understanding of the nanoscale architecture of anterograde synaptic transmission is rapidly expanding, the qualitative and quantitative molecular principles underlying distinct mechanisms of retrograde synaptic communication remain elusive. We show that a particular form of tonic cannabinoid signaling is essential for setting target cell-dependent synaptic variability. It does not require the activity of the two major endocannabinoid-producing enzymes. Instead, by developing a workflow for physiological, anatomical, and molecular measurements at the same unitary synapse, we demonstrate that the nanoscale stoichiometric ratio of type 1 cannabinoid receptors (CB1Rs) to the release machinery is sufficient to predict synapse-specific release probability. Accordingly, selective decrease of extrasynaptic CB1Rs does not affect synaptic transmission, whereas in vivo exposure to the phytocannabinoid Δ9-tetrahydrocannabinol disrupts the intrasynaptic nanoscale stoichiometry and reduces synaptic variability. These findings imply that synapses leverage the nanoscale stoichiometry of presynaptic receptor coupling to the release machinery to establish synaptic strength in a target cell-dependent manner.

Beyond the MUN domain, Munc13 controls priming and depriming of synaptic vesicles

Synaptic vesicle docking and priming are dynamic processes. At the molecular level, SNAREs (soluble NSF attachment protein receptors), synaptotagmins, and other factors are critical for Ca2+-triggered vesicle exocytosis, while disassembly factors, including NSF (N-ethylmaleimide-sensitive factor) and α-SNAP (soluble NSF attachment protein), disassemble and recycle SNAREs and antagonize fusion under some conditions. Here, we introduce a hybrid fusion assay that uses synaptic vesicles isolated from mouse brains and synthetic plasma membrane mimics. We included Munc18, Munc13, complexin, NSF, α-SNAP, and an ATP-regeneration system and maintained them continuously-as in the neuron-to investigate how these opposing processes yield fusogenic synaptic vesicles. In this setting, synaptic vesicle association is reversible, and the ATP-regeneration system produces the most synchronous Ca2+-triggered fusion, suggesting that disassembly factors perform quality control at the early stages of synaptic vesicle association to establish a highly fusogenic state. We uncovered a functional role for Munc13 ancillary to the MUN domain that alleviates an α-SNAP-dependent inhibition of Ca2+-triggered fusion.

Acute introduction of phosphoserine-129 α-synuclein induces severe swelling of mitochondria at lamprey synapses

Abnormal synaptic aggregation of α-synuclein is linked to cognitive deficits in Parkinson's disease (PD). While the impacts of excess α-synuclein on synaptic function are well established, comparatively less is known about the effects on local mitochondria. Here, we examined morphological features of synaptic mitochondria treated with wild type (WT) or phosphoserine 129 (pS129) α-synuclein, a variant with prominent synaptic accumulation in PD. Acute introduction of pS129 α-synuclein to lamprey synapses caused an activity-dependent swelling and bursting of mitochondria, which did not occur with WT α-synuclein. These pS129-induced effects on mitochondria likely contribute to the synaptic deficits observed in PD.

Lipid organization and turnover in the plasma membrane of human differentiating neural progenitor cells revealed by time-of-flight secondary ion mass spectrometry imaging

Membrane lipids have been known to influence multiple signalling and cellular processes. Dysregulation of lipids at the neuronal membrane is connected to a significant alteration of the brain function and morphology, leading to brain diseases and neurodegeneration. Understanding the lipid composition and turnover of neuronal membrane will provide a significant insight into the molecular events underlying the regulatory effects of these biomolecules in a neuronal system. In this study, we aimed to characterize the composition and turnover of the plasma membrane lipids in human neural progenitor cells (NPCs) at an early differentiation stage into midbrain neurons using ToF-SIMS imaging. Lipid composition of the native plasma membrane was explored, followed by an examination of the lipid turnover using different isotopically labelled lipid precursors, including 13C-choline, 13C-lauric acid, 15N-linoleic, and 13C-stearic. Our results showed that differentiating NPCs contain a high abundance of ceramides, glycerophosphoserines, neutral glycosphingolipids, diradylglycerols, and glycerophosphocholines at the plasma membrane. In addition, different precursors were found to incorporate into different membrane lipids which are specific for the short- or long-carbon chains, and the unsaturation or saturation stage of the precursors. The lipid structure of neuronal membrane reflects the differentiation status of NPCs, and it can be altered significantly using a particular lipid precursor. Our study illustrates a potential of ToF-SIMS imaging to study native plasma membrane lipids and elucidate complex cellular processes by providing molecular -rich information at a single cell level.

Vesicle and reaction-diffusion hybrid modeling with STEPS

Vesicles carry out many essential functions within cells through the processes of endocytosis, exocytosis, and passive and active transport. This includes transporting and delivering molecules between different parts of the cell, and storing and releasing neurotransmitters in neurons. To date, computational simulation of these key biological players has been rather limited and has not advanced at the same pace as other aspects of cell modeling, restricting the realism of computational models. We describe a general vesicle modeling tool that has been designed for wide application to a variety of cell models, implemented within our software STochastic Engine for Pathway Simulation (STEPS), a stochastic reaction-diffusion simulator that supports realistic reconstructions of cell tissue in tetrahedral meshes. The implementation is validated in an extensive test suite, parallel performance is demonstrated in a realistic synaptic bouton model, and example models are visualized in a Blender extension module.

Actin-nucleation promoting factor N-WASP influences alpha-synuclein condensates and pathology

Abnormal intraneuronal accumulation of soluble and insoluble α-synuclein (α-Syn) is one of the main pathological hallmarks of synucleinopathies, such as Parkinson's disease (PD). It has been well documented that the reversible liquid-liquid phase separation of α-Syn can modulate synaptic vesicle condensates at the presynaptic terminals. However, α-Syn can also form liquid-like droplets that may convert into amyloid-enriched hydrogels or fibrillar polymorphs under stressful conditions. To advance our understanding on the mechanisms underlying α-Syn phase transition, we employed a series of unbiased proteomic analyses and found that actin and actin regulators are part of the α-Syn interactome. We focused on Neural Wiskott-Aldrich syndrome protein (N-WASP) because of its association with a rare early-onset familial form of PD. In cultured cells, we demonstrate that N-WASP undergoes phase separation and can be recruited to synapsin 1 liquid-like droplets, whereas it is excluded from α-Syn/synapsin 1 condensates. Consistently, we provide evidence that wsp-1/WASL loss of function alters the number and dynamics of α-Syn inclusions in the nematode Caenorhabditis elegans. Together, our findings indicate that N-WASP expression may create permissive conditions that promote α-Syn condensates and their potentially deleterious conversion into toxic species.

Short-distance vesicle transport via phase separation

In addition to long-distance molecular motor-mediated transport, cellular vesicles also need to be moved at short distances with defined directions to meet functional needs in subcellular compartments but with unknown mechanisms. Such short-distance vesicle transport does not involve molecular motors. Here, we demonstrate, using synaptic vesicle (SV) transport as a paradigm, that phase separation of synaptic proteins with vesicles can facilitate regulated, directional vesicle transport between different presynaptic bouton sub-compartments. Specifically, a large coiled-coil scaffold protein Piccolo, in response to Ca2+ and via its C2A domain-mediated Ca2+ sensing, can extract SVs from the synapsin-clustered reserve pool condensate and deposit the extracted SVs onto the surface of the active zone protein condensate. We further show that the Trk-fused gene, TFG, also participates in COPII vesicle trafficking from ER to the ER-Golgi intermediate compartment via phase separation. Thus, phase separation may play a general role in short-distance, directional vesicle transport in cells.

SV2B defines a subpopulation of synaptic vesicles

Synaptic vesicles can undergo several modes of exocytosis, endocytosis, and trafficking within individual synapses, and their fates may be linked to different vesicular protein compositions. Here, we mapped the intrasynaptic distribution of the synaptic vesicle proteins SV2B and SV2A in glutamatergic synapses of the hippocampus using three-dimensional electron microscopy. SV2B was almost completely absent from docked vesicles and a distinct cluster of vesicles found near the active zone. In contrast, SV2A was found in all domains of the synapse and was slightly enriched near the active zone. SV2B and SV2A were found on the membrane in the peri-active zone, suggesting the recycling from both clusters of vesicles. SV2B knockout mice displayed an increased seizure induction threshold only in a model employing high-frequency stimulation. Our data show that glutamatergic synapses generate molecularly distinct populations of synaptic vesicles and are able to maintain them at steep spatial gradients. The almost complete absence of SV2B from vesicles at the active zone of wildtype mice may explain why SV2A has been found more important for vesicle release.

ATG9 resides on a unique population of small vesicles in presynaptic nerve terminals

In neurons, autophagosome biogenesis occurs mainly in distal axons, followed by maturation during retrograde transport. Autophagosomal growth depends on the supply of membrane lipids which requires small vesicles containing ATG9, a lipid scramblase essential for macroautophagy/autophagy. Here, we show that ATG9-containing vesicles are enriched in synapses and resemble synaptic vesicles in size and density. The proteome of ATG9-containing vesicles immuno-isolated from nerve terminals showed conspicuously low levels of trafficking proteins except of the AP2-complex and some enzymes involved in endosomal phosphatidylinositol metabolism. Super resolution microscopy of nerve terminals and isolated vesicles revealed that ATG9-containing vesicles represent a distinct vesicle population with limited overlap not only with synaptic vesicles but also other membranes of the secretory pathway, uncovering a surprising heterogeneity in their membrane composition. Our results are compatible with the view that ATG9-containing vesicles function as lipid shuttles that scavenge membrane lipids from various intracellular membranes to support autophagosome biogenesis.Abbreviations: AP: adaptor related protein complex: ATG2: autophagy related 2; ATG9: autophagy related 9; DNA PAINT: DNA-based point accumulation for imaging in nanoscale topography; DyMIN STED: dynamic minimum stimulated emission depletion; EL: endosome and lysosome; ER: endoplasmic reticulum; GA: Golgi apparatus; iBAQ: intensity based absolute quantification; LAMP: lysosomal-associated membrane protein; M6PR: mannose-6-phosphate receptor, cation dependent; Minflux: minimal photon fluxes; Mito: mitochondria; MS: mass spectrometry; PAS: phagophore assembly site; PM: plasma membrane; Px: peroxisome; RAB26: RAB26, member RAS oncogene family; RAB3A: RAB3A, member RAS oncogene family; RAB5A: RAB5A, member RAS oncogene family; SNARE: soluble N-ethylmaleimide-sensitive-factor attachment receptor; SVs: synaptic vesicles; SYP: synaptophysin; TGN: trans-Golgi network; TRAPP: transport protein particle; VTI1: vesicle transport through interaction with t-SNAREs.

Senktide blocks aberrant RTN3 interactome to retard memory decline and tau pathology in social isolated Alzheimer's disease mice

Sporadic or late-onset Alzheimer's disease (LOAD) accounts for more than 95% of Alzheimer's disease (AD) cases without any family history. Although genome-wide association studies have identified associated risk genes and loci for LOAD, numerous studies suggest that many adverse environmental factors, such as social isolation, are associated with an increased risk of dementia. However, the underlying mechanisms of social isolation in AD progression remain elusive. In the current study, we found that 7 days of social isolation could trigger pattern separation impairments and presynaptic abnormalities of the mossy fibre-CA3 circuit in AD mice. We also revealed that social isolation disrupted histone acetylation and resulted in the downregulation of 2 dentate gyrus (DG)-enriched miRNAs, which simultaneously target reticulon 3 (RTN3), an endoplasmic reticulum protein that aggregates in presynaptic regions to disturb the formation of functional mossy fibre boutons (MFBs) by recruiting multiple mitochondrial and vesicle-related proteins. Interestingly, the aggregation of RTN3 also recruits the PP2A B subunits to suppress PP2A activity and induce tau hyperphosphorylation, which, in turn, further elevates RTN3 and forms a vicious cycle. Finally, using an artificial intelligence-assisted molecular docking approach, we determined that senktide, a selective agonist of neurokinin3 receptors (NK3R), could reduce the binding of RTN3 with its partners. Moreover, application of senktide in vivo effectively restored DG circuit disorders in socially isolated AD mice. Taken together, our findings not only demonstrate the epigenetic regulatory mechanism underlying mossy fibre synaptic disorders orchestrated by social isolation and tau pathology but also reveal a novel potential therapeutic strategy for AD.

Structural Plasticity of GABAergic Pallidothalamic Terminals in MPTP-Treated Parkinsonian Monkeys: A 3D Electron Microscopic Analysis

The internal globus pallidus (GPi) is a major source of tonic GABAergic inhibition to the motor thalamus. In parkinsonism, the firing rate of GPi neurons is increased, and their pattern switches from a tonic to a burst mode, two pathophysiological changes associated with increased GABAergic pallidothalamic activity. In this study, we used high-resolution 3D electron microscopy to demonstrate that GPi terminals in the parvocellular ventral anterior nucleus (VApc) and the centromedian nucleus (CM), the two main GPi-recipient motor thalamic nuclei in monkeys, undergo significant morphometric changes in parkinsonian monkeys including (1) increased terminal volume in both nuclei; (2) increased surface area of synapses in both nuclei; (3) increased number of synapses/GPi terminals in the CM, but not VApc; and (4) increased total volume, but not number, of mitochondria/terminals in both nuclei. In contrast to GPi terminals, the ultrastructure of putative GABAergic nonpallidal terminals was not affected. Our results also revealed striking morphological differences in terminal volume, number/area of synapses, and volume/number of mitochondria between GPi terminals in VApc and CM of control monkeys. In conclusion, GABAergic pallidothalamic terminals are endowed with a high level of structural plasticity that may contribute to the development and maintenance of the abnormal increase in pallidal GABAergic outflow to the thalamus in the parkinsonian state. Furthermore, the evidence for ultrastructural differences between GPi terminals in VApc and CM suggests that morphologically distinct pallidothalamic terminals from single pallidal neurons may underlie specific physiological properties of pallidal inputs to VApc and CM in normal and diseased states.

Syntaxin1A overexpression and pain insensitivity in individuals with 7q11.23 duplication syndrome

Genetic modifications leading to pain insensitivity phenotypes, while rare, provide invaluable insights into the molecular biology of pain and reveal targets for analgesic drugs. Pain insensitivity typically results from Mendelian loss-of-function mutations in genes expressed in nociceptive (pain-sensing) dorsal root ganglion (DRG) neurons that connect the body to the spinal cord. We document a pain insensitivity mechanism arising from gene overexpression in individuals with the rare 7q11.23 duplication syndrome (Dup7), who have 3 copies of the approximately 1.5-megabase Williams syndrome (WS) critical region. Based on parental accounts and pain ratings, people with Dup7, mainly children in this study, are pain insensitive following serious injury to skin, bones, teeth, or viscera. In contrast, diploid siblings (2 copies of the WS critical region) and individuals with WS (1 copy) show standard reactions to painful events. A converging series of human assessments and cross-species cell biological and transcriptomic studies identified 1 likely candidate in the WS critical region, STX1A, as underlying the pain insensitivity phenotype. STX1A codes for the synaptic vesicle fusion protein syntaxin1A. Excess syntaxin1A was demonstrated to compromise neuropeptide exocytosis from nociceptive DRG neurons. Taken together, these data indicate a mechanism for producing "genetic analgesia" in Dup7 and offer previously untargeted routes to pain control.